This invention relates to a knocking control system for internal combustion engines, and particularly to a system of this kind for an internal combustion engine equipped with a device which varies the opening and closing timing or lift of inlet valves and exhaust valves of the engine depending on operating conditions of the engine in order to prevent knocking.
An internal combustion engine is conventionally known, e.g. from Japanese Patent Publication (Kokoku) No. 49-33289, which is capable of changing the valve timing (valve opening period or valve lift) of inlet valves and/or exhaust valves between low speed valve timing suitable for a lower engine rotational speed region and high speed valve timing suitable for a higher engine rotational speed region, in order to enhance the charging efficiency or combustion efficiency.
Further, a control method for internal combustion engines is known, e.g. from Japanese Patent Publication (Kokoku) 57-30980, in which knocking occurring in the engine is detected, and the ignition timing is retarded upon detection of knocking to thereby eliminate the knocking.
However, if knocking control is effected in an internal combustion engine equipped with the valve timing device, there occur the following problems:
Knocking is caused by abnormal combustion within engine cylinders. The magnitude (knocking noise level) and frequency of knocking, as well as the crank angle at which knocking occurs vary if the valve timing and/or lift of the inlet and exhaust valves and/or the compression ratio vary. Further, the noise level (background level) of the output of a knocking sensor, on the basis of which a knocking discrimination level is set, varies at the time of changeover of the valve timing when changeover of rocker arms and oil passages, ete. is effected. Also, the ignition timing at which knocking occurs and the optimal ignition timing (MBT) are different between the low speed valve timing and the high speed valve timing. Therefore, knocking is not eliminated to thereby cause damage to the engine and degraded driveability and hence marketability of same, unless the retarding amount, retarding speed, advancing amount, and advancing speed of the ignition timing are set to respective appropriate values depending on the actual valve timing when the ignition timing is retarded upon detection of knocking to eliminate same and when it is advanced after elimination of the knocking.
In order to solve the above problems, if the values of discrimination parameters used by knocking discriminating means or the values of control parameters used by knocking control means are varied in accordance with changeover of the valve timing responsive to engine operating conditions, knocking control can be properly carried out irrespective of the actual valve timing. However, if a failure occurs in the valve timing control means in such a case, the values of the discrimination parameters or the control parameter cannot be varied correctly in accordance with changeover of the value timing, resulting in occurrence of knocking.